1) Field of the Invention
This invention relates to an engine output controlling method for a drive-by-wire (hereinafter called "DBW") control system which drives and controls the degree of opening of a throttle valve by a motor.
2) Description of the Related Art
Heretofore it has been a common knowledge to control the output of a gasoline engine for automobile by varying the amount of intake air by regulating the degree of opening of a throttle valve disposed in an intake air passage.
The throttle valve is operatively connected to an accelerator pedal to be operated by the driver. In an electronically controlled automobile engine, the throttle valve is electronically driven by an electric motor; that is, a drive-by-wire (DBW) control system is adopted so that the degree of opening of the throttle valve can be controlled independently, apart from the operation of the accelerator pedal according to need.
However, in the conventional DBW control system, the degree of opening of the throttle valve is controlled in linear proportion to the operation of the accelerator pedal unless the throttle valve is released from the operation of the accelerator and is then controlled by the electric motor on priority basis. In other words, in controlling the throttle valve based on the operation of the accelerator, the degree of opening of the throttle is controlled in proportion to the operation of the accelerator pedal like a throttle control system of the mechanical link type in which the accelerator pedal and the throttle valve are directly connected by a wire or a link.
In general, the throttle valve is to vary the area of opening of the intake air passage to change the amount of intake air. But, as is understood from the contour of the intake air passage and the structure of the throttle valve, the degree of opening of the throttle valve is not proportional to the area of opening of the intake air passage.
Therefore the degree of opening of the throttle valve and the amount of intake air are not linearly proportional to each other; that is, the state of operation of the accelerator pedal is not necessarily proportional to the output of the engine.
Consequently, it has hitherto been customary for the driver to operate the accelerator pedal by the rule of trial and error or in his/her estimation using the instinct on experiences.
However, in the conventional DBW control system, by utilizing the characteristic that the throttle valve is controlled individually by the electric motor, it is possible to read the required output of the engine from the state of operation of the accelerator and then to vary the degree of opening of the throttle valve so as to obtain a target amount of intake air that causes this required engine output.
In that case, yet another method of controlling the throttle valve could be adopted to read, from the position of depression of the accelerator pedal, an engine torque target or a drive shaft torque target as the required value, and then to obtain the most suitable engine output so as to create a desired engine torque or drive shaft torque from the position of depression of the accelerator pedal.
With that arrangement, a desired engine torque or drive shaft torque can be obtained readily from the position of depression of the accelerator pedal, causing improved response. But in controlling an engine torque target or a drive shaft torque target from the position of depression of the accelerator pedal, a predetermined position of depression of the accelerator pedal will produce a predetermined amount of torque so that a constant engine torque can be created if the position of depression of the accelerator pedal is kept constant when the vehicle is running on a downhill road, The vehicle is thereby accelerated.
Specifically, in the case where the vehicle is running on a flat load with the position of the accelerator pedal remaining constant, because the output engine torque is constant, if the transmission gear ratio and the vehicle resistance are constant, the running speed of the vehicle converges to a constant speed using the full engine torque without creating any surplus.
Nonetheless, when the vehicle is shifted to a downhill running with the position of depression of the accelerator pedal, the output engine torque is constant, and instead such large torque is not needed for a downhill running so that an excess torque is created to make an acceleration with as time passes.
For converging such acceleration to a predetermined value, the depressed accelerator pedal have to be returned to some extent to reduce the degree of opening of the throttle valve so as to lower the engine output.
However, in the throttle control system of the mechanical link type in which the accelerator pedal and the throttle valve are directly connected to each other by a wire or a link, as is apparent from the characteristic graph of FIG. 4, if the position of depression of the accelerator pedal is constant, the output of the engine is automatically lowered as the engine speed rises. Therefore in downhill running, the engine torque is lowered as the number of rotations of the engine (=vehicle speed) rises without returning the depressed accelerator pedal, thus causing the engine brake to start working. Consequently in downhill running, if the position of operation of the accelerator pedal is kept constant, the vehicle speed can converge to a predetermined value.
From the foregoing, if arrangements are set to produce a constant torque by a constant amount of depression of the accelerator pedal in the DBW control system, it is necessary to take the trouble to return the accelerator pedal when the vehicle speed rises in downhill running. To the contrary, such returning is unnecessary in the case of a throttle control system of the mechanical link type. So if the vehicles with these two systems are driven respectively, different driving ways are required as discussed above, thus each giving a different touch to the driver.